Tunable filters are important elements in integrated optical transmission and switching systems. Specifically, tunable filters at the receiver end of such systems can be used to selectively separate each of the multi-wavelengths sent by the transmitters. In a multi-wavelength transmission system, such transmitters can be made of either multi-wavelength laser arrays or tunable lasers. These multi-wavelength laser arrays or tunable lasers can be used to impart modulated signal chains on different wavelength carriers and these signals are multiplexed into a single mode optical fiber. At the receiver end these wavelength division multiplexed (WDM) signals are demultiplexed using a wavelength tunable filter in conjunction with appropriate detectors. The tunable filter which may preferably be in the form of a directional coupler extracts an optical signal of a predetermined wavelength from a plurality of wavelengths propagating through a waveguide.
For a given wavelength propagating through one of a pair of adjacent waveguides there is one set of conditions whereby all of that wavelength will be coupled to the adjacent waveguide. This characteristic is well documented. See for example, D. Marcuse, "Bandwidth of Forward and Backward Coupling Directional Couplers", Journal of Lightwave Technology, Vol. LT.5, NO. 12, December 1987, at page 1773.
It is also known that an optical signal travelling through an optical fiber is subject to variations in polarization state due to the birefringence of conventional single mode optical fibers. Therefore, an optical signal of a given wavelength .lambda..sub.0 with single polarization state will have associated with it both TE and TM polarization modes, i.e., .lambda..sub.0 (TE) and .lambda..sub.0 (TM) after travelling some distance through a fiber. Therefore, it is impossible for a narrow band filter of the prior art to selectively couple both TE and TM modes from one waveguide to the other, since the coupling wavelengths of TE and TM modes are separated as much as 30 nm in such a directional coupler filter with a -3dB bandwidth of 1.5 nm. [See Chi Wu et al, "InGaAs/InP Vertical Directional Coupler Filter with Optimally Designed Wavelength Tunability", IEEE Photonics Technology Letters, Vol. 4, No. 4, April 1993.] For complete and effective coupling of a polarized optical signal with wavelength .lambda..sub.0, the following three conditions must be satisfied. These are:
(1) Common central wavelength .lambda..sub.0 --in other words the central wavelength of the TE mode .lambda..sub.0.sup.TE must be equal PA0 (2) The bandwidth at the -3db points for both modes must be equal, i.e., .DELTA..lambda..sub.BW (TE)=.DELTA..lambda..sub.BW (TM) and; PA0 (3) The coupling coefficient for the TE mode must be the same as the coupling coefficient of the TM mode, i.e., K(TE)=K(TM).
to the central wavelength of the TM mode (.lambda..sub.0.sup.TE =.lambda..sub.0.sup.TM).